Two deflection laws of four-wheel steering technology
Generally speaking, according to different driving conditions, four-wheel steering technology can make the front and rear wheels have different steering angles, which is helpful for vehicles to turn or change lanes better.
When turning at low speed or the steering wheel angle is large, the front and rear wheels turn in opposite directions, and the rear wheel angle usually increases within a certain range (generally 5) with the increase of the steering angle, thus reducing the turning radius of the vehicle. In the complex environment of off-road roads, narrow paths, parking and other scenes, reverse steering can effectively improve vehicle flexibility.
At high speed, the four-wheel steering technology is changed to in-phase steering, and the deflection angle remains unchanged. However, due to the increase of vehicle speed at this time, the rear wheel deflection range is much smaller than that of reverse steering (generally 1). Vehicles with in-phase steering can reduce the yaw rate of the car body at high speed, effectively restrain the trend of dynamic lateral deviation of the car body and improve the handling stability.
The early four-wheel steering technology only had the mechanical structure of angle follow-up, and the front and rear wheels were always in reverse steering. Because this state is only suitable for low-speed scenes and seriously affects high-speed stability, it is only used in some military vehicles and engineering vehicles. With the development of later technology, the speed sensing four-wheel steering technology has been developed, which can realize two steering laws: reverse steering and in-phase steering according to different vehicle speeds.
Why can't four-wheel steering technology be popularized?
Compared with ordinary front-wheel steering vehicles, four-wheel steering technology can effectively improve low-speed flexibility and high-speed stability, which seems to be a perfect solution. However, because this effect can only be achieved by switching two different deflection laws, it is necessary to accurately judge the road scene and the driver's intention.
The traditional four-wheel steering technology is mainly based on speed judgment, low-speed anti-phase deflection and high-speed in-phase deflection, but the way is still too rough and there are still disadvantages.
This is also the biggest problem that restricts the development of four-wheel steering technology at present, so the four-wheel steering system used in passenger cars usually gives up anti-phase deflection and focuses on improving high-speed stability. At low speed, the car only relies on the front wheel to steer, and after the speed exceeds a certain limit, the rear wheel will participate in the in-phase deflection. However, due to the limited application scenarios of this scheme, the current mainstream electronically controlled hydraulic power system is still limited in flexibility and accuracy and has not been popularized.
Toyota's four-wheel steering technology
According to Toyota's patent information, the four-wheel steering patent applied for this time is to install an independent motor on each wheel, so that the maximum rotation angle of a single wheel can reach 90, and two deflection laws of in-phase steering and anti-phase steering can be realized. Compared with the electronic control hydraulic power system, the motor used in it also has better performance in control performance, system layout, energy saving and so on.
Different from the traditional four-wheel steering system, Toyota's patent is no longer limited to assisting in improving low-speed flexibility and high-speed stability, but has realized more flexible scene applications. The patented wheel angle range is greatly improved, and the front and rear wheels can be deflected by 90, that is to say, they can turn around or translate to the parking space, which is more applicable in some special road conditions and parking scenes. In addition, Toyota's patent also supports the deflection of four wheels at the same angle when driving obliquely.
In order to cooperate with the special function of this four-wheel steering system, Toyota also applied for a new type of acousto-optic prompt signal in the patent, which tells other traffic participants the next driving direction through special regular light flashing or image information.
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The most difficult part of four-wheel steering technology is not the structural design and electronic system, but how to accurately judge the driver's intention. Different from the traditional four-wheel steering technology, Toyota's new four-wheel steering technology not only retains the ability to optimize the dynamic performance of vehicles, but also provides more driving modes for vehicles and solves more practical needs. Of course, this still has the same problem. If the four-wheel steering technology is to be truly popularized, it still needs to wait for the arrival of the era of high-order automatic driving.
This article comes from car home, the author of the car manufacturer, and does not represent car home's position.